Enhanced 911 system for providing witness identification in a wireless communication system

ABSTRACT

The provisioning of enhanced 911 service in a mobile communications network is supplemented to include the capability of identifying other mobile stations in close proximity to a mobile station placing a 911 call. This capability may then be used to aid in the identification of witnesses to a crime, car accident, and the like. Constantly updated location information for mobile stations in communication with a switching center is maintained in a database that can be accessed by PSAP agents on an “as needed” basis. In particular, a PSAP agent can submit a search request, using time/date and geographic location information to determine the identity of all mobile stations in a predetermined “radius” associated with a 911 caller.

TECHNICAL FIELD

The present invention relates to the provisioning of enhanced 911service in a wireless communication environment and, more particularly,to the additional capability of locating potential witnesses in terms ofother cell phone users, in response to certain cell phone-based 911calls.

BACKGROUND OF THE INVENTION

Wireless telephones have received wide acceptance for use in cellularsystems, as well as in wireless user premises equipment applications.There are new cellular telephone systems under development, as well aswireless personal communication systems (PCS) for both the licensed andunlicensed bands. A cellular telephone or cell-like communication systeminvolves a network of fixed base stations that provide an integratedcommunication service to a plurality of mobile transmitter/receiver(“transceiver”) units, e.g., cellular telephones. The communicationsnetwork attempts to communicate with each transceiver from the basestation that provides the optimal communication, such as in terms ofsignal level, clarity, etc. The optimal base station is usually, but notnecessarily, the one nearest the mobile transceiver. To provide theoptimal communications support, the network needs to locate thegeographic position of the mobile only to the “rough” level required toassign the proper base station.

This rough estimate of the location of a mobile has been a hindrance inextending the conventional 911 aspect of communication service to themobile environment. When a user makes an emergency 911 call on astandard corded telephone, the location of the user is quicklydetermined since the physical location of the telephone is known andunchanging. In contrast, cell phone callers can “roam” anywhere withinthe physical bounds of the entire system and, as result, “permanent”geographic location information associated with the cell phone ismeaningless.

Based on this realization, the FCC has defined an “Enhanced 911”requirement which mandates that all wireless service operators must beable to provide geographic position data to Public Service AnsweringPositions (PSAPs) for E911 calls by October 2001. As a result of thismandate, wireless network operators connecting to the public switchedtelephone network must implement E911 service in two phases: Phase Istipulates that the system must pass the caller's phone number,cell-site, and cell-sector location information through to a PSAP.Carriers were to complete this step by April 1998, but many are still inthe implementation stage. Phase II presents the more challenging task(at least from a location technology standpoint), of providing the 911caller's location to the appropriate PSAP with an accuracy of 125 metersRMS (root-mean-square), in at least 67% of all cases. As most wirelessoperators proceed to fulfill Phase I requirements through theirnetworks, they are also assessing which location technologies mosteffectively meet the Phase II requirements. These positioning methodsare generally divided into two categories: (1) network-based systemsthat require some equipment installation at network base stations; and(2) handset-based systems that add GPS or another location technology tothe wireless phone, but generally do not require additional networkequipment.

Most network-based caller-location systems are based ontime-difference-of-arrival (TDOA) or angle-of-arrival (AOA)measurements, or a combination of these two techniques. Cell-Locprovides one exemplary technology to determine the geographic positionof mobile stations, as disclosed in U.S. Pat. No. 5,890,068, issued toM. T. Fattouche et al. on Mar. 30, 1999. In this case, receive-onlyantennas (ROAs) are located at base stations and TDOA measurements aremade for various channel transmissions from mobile stations. Thisinformation is then used to determine the position of the devices,without requiring alterations to either the base stations or the mobilestations. In AOA technology, a set of receive-only phased array antennasare located at each base station and used to compute the angle at whichsignals transmitted from a mobile station arrive at the base station.See, for example, U.S. Pat. No. 5,786,791, entitled “Method fordetermining an angle of arrival of a signal transmitted by a remote unitin a communication system”, issued to E. J. Bruckert on Jul. 28, 1998. Adisadvantage of both of these techniques is that the geographic positionof a mobile station can only be determined when the mobile station istransmitting. An enhancement to these network-based techniques, referredto as RF fingerprinting, measures the distinct RF patterns and multipathcharacteristics of radio signals arriving at a cell site from a mobileunit, using this information to determine the mobile's location with anadditional level of granularity.

The FCC mandate clearly requires that wireless carriers be able tolocate any caller requesting emergency assistance through its network.On the surface, this would appear to eliminate handset-based solutions,such as utilizing a Global Positioning System (GPS), from considerationsince it would be essentially impossible to add GPS (or otherlocation-sensitive components) to all phones operating on a network byOctober 2001. However, it is presumed that such a system could be phasedin, with newer phones including the necessary technology. At a recentconference on wireless location system implementation, a Nokiarepresentative reported that the company currently favors adding GPS tothe handset as the best solution for caller location on CDMA networks.Ericsson has suggested a short-term solution based on TDOA and along-term solution combining GPS in the phone with differentialcorrections, using a network server. In addition to manufacturingwireless phones, both Nokia and Ericsson supply wireless networkinfrastructure equipment. Lucent Technologies and Qualcomm, which alsomanufacture both wireless handsets and infrastructure equipment, reportthat they too are investigating long-term caller location solutions thatinclude the addition of GPS to handsets. These wireless infrastructuresuppliers generally favor GPS as an element of the long-term solutionbased on the view that aided-GPS will support a higher level accuracywill be needed to support a wide range of commercial location-basedservices. However, a number of major carriers continue to express apreference for a network-based solution.

Regardless of the technology ultimately deployed to provide the positionlocation information of a mobile E911 caller, there are further aspectsof this technology that may be deployed to provide additional featuresto such a system.

SUMMARY OF THE INVENTION

A need remaining in the prior art is addressed by the present invention,which relates to the provisioning of enhanced 911 service in a wirelesscommunication environment and, more particularly, to the additionalcapability of locating potential witnesses in terms of other cell phoneusers, in response to certain cell phone-based 911 calls.

In accordance with the present invention, an off-line “position server”is added to the communication network and, upon receiving a request froma PSAP, will determine the identity and location of various otherwireless communication devices in the vicinity of the 911 caller. Thus,in instances where it may be important to find witnesses (for anaccident, a robbery, or the like), the stored location informationassociated with various other wireless devices in the network may beretrieved and those individuals contacted as potential witnesses.

In the practice of the present invention, an E911 agent at a PSAP willdetermine those calls which would benefit from “witness” information andthen, on a case-by-case basis, launch a query to the position server tofind and store this information (for later retrieval by the police,investigating entity, or other authorized individuals). The queryincludes a specific “radius” and time/date in the request, such as “findall cell phone callers within a two mile radius of latitude 29° E,longitude 110° N at 2PM EDT”. Since the information regarding thecapability to locate each wireless device is being developed, theability to store and then retrieve this information will be helpful inidentifying people that can assist in various emergency situations.

In one embodiment of the present invention, the position server itselfmay contain a database for storing the location information of allmobile stations in the communication system. In this case, the positionserver directly searches the database for the identity of mobilestations that would satisfy a particular search request from a PSAP, andstores the results in a table in the position server.

In an alternative embodiment, a network element (such as a gatewayelement between the wireless network and a traditional PSTN) may storethe location information for a set of mobile stations that communicatesthrough that particular gateway (also referred to, for example, as amobile switching center). In this arrangement, when the position serverreceives a request from a PSAP to locate potential witnesses for a 911call (referred to in this discussion as a “snapshot request”), therequest is then forwarded to the relevant gateway elements that would bestoring location information for mobile stations in the requestedterritory. Each gateway element then searches its own database andforwards the results to the position server, which then collects all ofthe incoming information and forms a “transaction table”, for thatparticular “snapshot request”, the table storing all of the relevantinformation associated with the mobile stations in the vicinity of the911 call.

In yet another embodiment, each mobile station may be equipped with aninternal location buffer for automatically determining and storing thelocation of the mobile station at any given time. In this arrangement,when the position server receives a “snapshot request”, the request isagain forwarded to the relevant gateway elements (e.g., mobile switchingcenters), which then broadcast the request to all mobile stations in itsserving area. The mobile stations then check their current geographiclocation information against the location information in the request,and forward their unique identity information to the network element ifthey are indeed in the relevant area of the search.

Other and further embodiments of the present invention will becomeapparent during the course of the following discussion and by referenceto the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings,

FIG. 1 illustrates, in simplified block diagram form, an exemplary priorart Global System for Mobile Communication (GSM) that may be modified toincorporate the teachings of the present invention;

FIG. 2 is a diagram of the protocol stack associated with the GSM systemof FIG. 1;

FIG. 3 illustrates a portion of the GSM network that has been modifiedto include a first embodiment of the present invention, utilizingreceive-only antennas to assist in locating witnesses to an E911 callfrom a mobile station;

FIG. 4 contains an exemplary message sequence associated with theimplementation of the present invention in the network of FIG. 3;

FIGS. 5 and 6 contain flowcharts illustrating the function of anexemplary position server in the working of the present invention;

FIG. 7 illustrates an exemplary table for storing current mobile stationlocation data in accordance with the present invention;

FIG. 8 contains a graphical illustration of an exemplary “witnesslocation” “snapshot request” performed in accordance with the presentinvention;

FIG. 9 is an exemplary “transaction table” of all identified mobilestations (i.e., potential witnesses), as being within a defined boundaryof a particular 911 call;

FIG. 10 illustrates an alternative embodiment of the present invention,utilizing a network element, such as a mobile switching center, to storegeographic location information for a plurality of mobile stations;

FIG. 11 illustrates a particular message sequence associated with thenetwork architecture of FIG. 10;

FIG. 12 contains a flowchart of the process used by a mobile station tosend its geographic location information to its associated mobileswitching center;

FIG. 13 is an exemplary table that may be used by a mobile switchingcenter to store the location information received from the plurality ofmobile stations in its serving area;

FIG. 14 contains a flowchart of a position server process for respondingto a “snapshot request” from a PSAP;

FIG. 15 contains a flowchart of the mobile switching center process forresponding to a “snapshot” request from the position server;

FIG. 16 comprises a database of relevant identification information forall mobile switching centers in the communication network, stored in theposition server;

FIG. 17 illustrates an exemplary message sequence associated with analternative process where each mobile station stores its currentlocation in an internal buffer;

FIG. 18 is a flowchart of a process for updating the locationinformation at a mobile station; and

FIG. 19 is a flowchart of a process used by a mobile station to respondto a “snapshot request” broadcasted by a mobile switching center.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary prior art Global System for MobileCommunications (GSM) network 10 that is useful to understand prior todiscussing the implementation of various embodiments of a “witnesslocator” service in accordance with the present invention. As shown, aplurality of mobile stations 12 are dispersed throughout the network,where various subsets of stations 12 are in communication with aplurality of base stations subsystems 14. One exemplary base stationsubsystem 14 is shown in slightly more detail as comprising a basetransceiver station 16 and a base station controller 18. It is to beunderstood that each base station subsystem 14 includes similarcomponents. As shown in FIG. 1, base station controller 18 is incommunication with a mobile switching center 20, where a plurality ofmobile switching centers 20 function as gateway elements between thewireless communication system and a conventional telecommunicationsnetwork 22, such as the Public Switched Telephone Network (PSTN).Associated with each mobile switching center 20 is a “home” locationregister 24 and a “visitor” location register 26, which communicate witheach other to keep track of the various mobile units 12 (each mobilecomprising a unique ID) and whether the particular mobile switchingcenter 20 is their “home” center 20 or, alternatively, whether thatparticular mobile unit has roamed and is therefore “visiting” anotherswitching center 20.

Further illustrated in network 10 of FIG. 1 is a plurality of PublicSafety Answering Positions (PSAPs) 30, used to respond to 911 callsplaced by the mobile devices 12. Each “position” at a PSAP location mayinclude both telephones 32 and computer terminals 34, to aid the agentsmanning PSAPs 30 in responding to received distress calls. Thearchitecture of network 10, as illustrated in FIG. 1, can be consideredas the current state of the art in terms of the interaction of thecomponents and the ability of a PSAP 30 to respond to a 911 call placedby a mobile station 12.

The various protocols used to communicate throughout GSM network 10 areillustrated in the model of FIG. 2. Included within the protocol stack40 at mobile station 12 is a “mobility management” protocol 42, where MMprotocol 42 can be enhanced, in accordance with the present invention,to transmit the geographic location of mobile station 12 to its mobileswitching center 20 (see MM protocol 44 within protocol stack 46associated with mobile switching center 20). MM protocol 42 is primarilyconcerned with location, registration and security information.Conventional location updating occurs when mobile station 12 beings anew call. Periodic location updating happens at regular intervals, wherethe main purpose of MM protocol 42 is to maintain the integrity of the“home” and “visitor” location registers 24 and 26, respectively. Inparticular, a “Location Updating Request” message is transmitted from agiven mobile station 12 to its associated mobile switching center. 20.In accordance with the present invention, this Location Updating Requestmessage is enhanced to contain the geographic position (viz., latitude,longitude) of mobile station 12.

FIG. 3 illustrates a portion of an exemplary GSM network that has beenmodified to include the capability of identifying potential witnessesfor various emergency situations, in accordance with the presentinvention. As shown in this particular embodiment, base stationsubsystem 14 has been modified to include a receive-only antenna 50,co-located with base transceiver station 16 so as to simultaneouslyreceive each transmission from each mobile station 12 in communicationwith that particular base station subsystem 14 (it is to be noted thateach base station subsystem 14 in the network is similarly modified toinclude such a receive-only antenna 50). An exemplary transmission, inaccordance with the GSM protocol model, will include (among other piecesof information) the particular cell phone number associated withtransmitting mobile station 12 and its current location. Basetransceiver station 16 uses this information in the normal fashion tofacilitate communication between mobile station 12 and its desiredreceiving station (not shown). Receive-only antenna 50 collects thissame information and forwards it to a position server 52. As will bediscussed in detail below, position server 52 functions to store thegeographic position of the last transmission for each mobile station 12associated with base station subsystem 14, and then use this informationto respond to “snapshot requests” from a PSAP regarding the identity ofpotential witnesses for a particular 911 call.

In accordance with the present invention, when an agent at a PSAP 30believes that it would be useful to capture potential witnessinformation for a particular 911 call, the agent will launch a query,through a data network 54, to position server 52, requesting positionserver 52 to collect the identity of all cell phones whose last-recordedgeographic distance is within a certain boundary of the geographiclocation of the 911 caller (the boundary, usually defined by a radiussurrounding the origination site of the 911 call, can be defined by the911 agent). The information collected by position server 52, in responseto this query, may then be stored as an identifiable table withinposition server 52. Later on, the police (or any other authenticatedinvestigative agency) can retrieve the information and contact theindividuals associated with the cell phone numbers that were collected.For example, if a “hit and run” car accident occurred at a particularintersection, the PSAP agent can launch a query to position server 52,requesting position server 52 to “snapshot” all cell phones within twoblocks of the scene of the accident, during the time of the accident.Such a request may result in, for example, a response that ten differentcell phones were in use in the vicinity of the accident. The police canthen contact the individuals associated with these cell phones todetermine if any of them witnessed the accident and can supply anydetails to aid in the investigation.

FIG. 4 illustrates the message sequence associated with thecommunication steps described thus far. As shown in line “1”, a “voicecommunication”—in this case, a 911 call, is first established between agiven mobile station 12 and a PSAP agent 30. At the same time, thepositional information related to mobile station 12 is recovered byreceive-only antenna 50 (co-located with base station controller 18) andtransmitted to position server 52, denoted as line “2” in FIG. 4. In apreferred embodiment, the message format for such a “geographicposition” message would include the following information: (1) telephonenumber of the mobile station; (2) latitude; (3) longitude; and (4) time.When and if the agent at PSAP 30 determines that it would be useful tolearn the identity of other cell phone users in the vicinity of a 911call in progress, PSAP 30 will launch a “snapshot request” query toposition server 52 (line “3” in FIG. 4), where this query may have theformat of: (1) telephone number of 911 caller; (2) telephone number of911 agent; (3) latitude of 911 caller; (4) longitude of 911 caller; (5)radius of requested search. In return, position server 52 willacknowledge receipt of the request back to PSAP 30 (line “4” in FIG. 4),the acknowledgement carrying a unique snapshot ID for this request.

FIGS. 5 and 6 contain flowcharts illustrating the two differentfunctions of an exemplary position server 52. FIG. 5 shows, inparticular, the steps associated with updating the current positioninformation of an exemplary mobile station 12. When the process firststarts, “geographic position” information (as defined above) is receivedfrom mobile station 12 (block 60). The particular portion of the database associated with this mobile station is found, and the data stored(block 62). FIG. 7 illustrates an exemplary table for storing this data.In this particular arrangement, the stored data comprises the telephonenumber, date and time of the geographic position measurement and thelatitude and longitude of the mobile station location.

Referring to FIG. 6, the flowchart associated with the process of PSAP30 querying a position server 52 is shown. The process begins withreceiving a “snapshot” request at position server 52 (block 64). Asdiscussed above, the snapshot request will include the particular radiusR the server should use in conducting its search, the radius beingdetermined by the PSAP agent. The request will include the currentlatitude and longitude of the 911 caller (or other geographic locationidentifying information), and will be used by position server 52 todetermine the boundaries for the search (using the 911 caller locationas the center point). Position server 52 will then search through thetable as shown in FIG. 7, looking for all reporting mobile stationswithin the determined boundaries (block 66). FIG. 8 illustratesgraphically one such request, where a mobile station 12 (denoted “X”)placing the 911 call, with telephone number XXX-YYY-ZZZZ, is used as thecenter point, and all mobile stations within radius R are found. Threesuch mobile stations, designated A, D, and G are illustrated as fallingwithin this radius, with another mobile station, designated L, beingbeyond the designated boundary. Once mobile stations are identified, thepertinent information about these stations is copied into a “transactiontable”, such as the table illustrated in FIG. 9 (block 68). Uponcompletion of the table, a “transaction ID”, unique to that request, iscreated (block 70) and transmitted back to PSAP 30 (block 72) as anacknowledgement that the request has been completed. At a later time,the police or other investigators may retrieve this information fromposition server 20, using the transaction ID stored at PSAP 30, andcontact the potential witnesses using the stored mobile stationtelephone numbers.

FIG. 10 illustrates an alternative embodiment of the present invention,where in this case each mobile station 12 automatically updates itsposition, storing this information at its associated mobile switchingcenter 20. This arrangement utilizes a conventional signaling messagethrough base station subsystem 14 without the need for an additional;receive-only antenna at subsystem 14. In one example, a system such asGPS can be used to update the position of each mobile station 12.Referring back to FIG. 2, mobility management (MM) protocol 42 as usedby mobile station 12 may be enhanced to provide this locationinformation. Currently, the MM protocol is concerned with informationrelated to location, registration and security. Standard locationupdating occurs in the prior art when the mobile station moves to a newcell, with periodic location updates at regular intervals. This use ofthe MM protocol is helpful in maintaining the integrity of HLR database24 and VLR database 26. In use, a “location updating request” message istransmitted from mobile station 12 to mobile switching center 20. Inaccordance with the present invention, the “location updating request”message is enhanced to include geographic information (for example,latitude and longitude).

Referring now to both FIGS. 10 and 11, the message sequence associatedwith this embodiment will be described in detail. An aspect of thisembodiment of the present invention is that the “location update” of thegeographic position of a mobile happens separately and independent ofany voice communication traffic between the mobile station and thecommunication system. FIG. 11 illustrates as message sequence (1) a“geographic position” update being sent directly from a mobile station12 to its associated mobile switching center 20. At a later point intime, a voice communication (911 call) is initiated by mobile station12, illustrated as message sequence (2) in FIG. 11. As with theembodiment described above, this communication is received by an agentat a PSAP 30. When and if the PSAP agent determines that it would beuseful to find “witnesses” in the area of the 911 call, the agentlaunches a “snapshot request”, through data network 54, to positionserver 52 (message sequence (3) in FIG. 11). In a preferred embodiment,the “snapshot request” message format includes the followinginformation: (1) telephone number of the 911 caller; (2) latitude of the911 caller; (3) longitude of the 911 caller; (4) radius, R, of searchzone; and (5) telephone number of the 911 agent. At this point, theprocess sequence is essentially identical to that described above.However, in this arrangement position server 52 does not store thegeographic information. Therefore, position server 52 will send anacknowledgement (message sequence (4)) back to PSAP 30 that the requesthas been received and assigned a unique transaction ID. Position server52 will then send a copy of the snapshot request (along with thetransaction ID) to one or more mobile switching centers 20 through datanetwork 54 (message sequence (5) in FIG. 11). The number and identity ofthe particular mobile switching centers that are queried depends upon,among other things, the geographic location of mobile station 12 placingthe 911 call, as well as the radius, R, of the search request entered bythe agent at PSAP 30. The identity of all mobile stations satisfying therequest will be sent as a “snapshot response” message (message sequence(6)) to position server 52, where position server 52 will hold thisinformation until later accessed by the authorized agencies requestingsuch information.

As mentioned above, this particular embodiment envisions the use ofmobile stations that can constantly update their geographic locationinformation, regardless of whether or not the mobiles are currentlyinvolved in an on-going voice communication session. FIG. 12 contains aflowchart illustrating the process used by an exemplary mobile station12 to provide this information. In one embodiment, each mobile stationmay be equipped with a GPS transmitter so as to automatically update itsgeographic location information. Referring to FIG. 12, the processbegins with mobile station 12 determining its current geographicposition (block 80). In an exemplary embodiment, the mobile's latitudeand longitude information may be used to define its geographic location.Next, the current time is determined (block 82). Mobile 12 station theninserts this information into the “mobility management” (MM) protocol 42and transmits this information upstream to its associated mobileswitching center 20 (block 84). Mobility switching center 20 then checksthe identity of the sending mobile station 12 and updates the geographicinformation in its database. An exemplary table useful for storing thisgeographic information at mobility switching center 20 is shown in FIG.13. In particular, the table stores the identity of each mobile station12 associated with mobility switching center 20, along with thegeographic location information (such as latitude and longitude) and thedata and time the last update was performed. This information can berefreshed at predetermined intervals. For example, referring again tothe flowchart of FIG. 12, the mobile station will “wait” for a period ofS seconds (block 86), as determined by either the mobile or thecommunication system, and then re-determine its geographic locationinformation (i.e., the process returns to block 80), again transmittingthe updated information to mobile switching center 20.

FIG. 14 contains a flowchart illustrating the process of responding to a“snapshot request” from the viewpoint of an exemplary position server52. As shown, the process begins with a PSAP 30 transmitting a “snapshotrequest” over (for example) data network 54 to position server 52 (block90). The request comprises the format discussed above, including thenecessary cell phone numbers as well as the geographic location of the911 caller. Position server 52 then assigns a unique transaction ID tothe request (block 102), and transmits this transaction ID as anacknowledgement to PSAP 30 (block 104).

Position server 52 next determines the mobile switching center (orcenters) 20 that would likely cover the geographic area associated withthis snapshot request (block 106), and forwards a copy of the “snapshotrequest”, with the transaction ID, to each relevant mobile switchingcenter (block 108) 20. This step is also denoted with an “A” in FIG. 14,and refers to the flowchart in FIG. 15, discussed below, which includesthe sequence of operations at a queried mobile switching center 20.Continuing on with the description of FIG. 14, the collected dataregarding potential witnesses is returned as a “snapshot response”message from each queried mobile switching center 20 (block 110),denoted with the letter “B” in FIG. 14. The received information is thenstored in a “snapshot table” at position server 20 (block 115), such asthe table illustrated in FIG. 9 and discussed above with the priorembodiment. Besides this database, and a database of all outstanding“snapshot requests”, position server 52 includes a database of relevantinformation about each mobile switching center 20 with which it maycommunicate. One such exemplary database is illustrated in FIG. 16,which includes identification information for each mobile switchingcenter 20, its network address and geographic information that can beused to “bound” the coverage area of the mobile switching center. Inthis example, the “corner” latitude and longitude information isrecorded (i.e., the “northwest”, “southwest”, “northeast”, and“southeast”) and used to define the geographic boundaries.

The process at each mobile switching center 20, as depicted in FIG. 15,begins with the reception of the “snapshot request” message from aposition server (block 120). Mobile switching center 20 then performs acheck through each entry in its database, using the geographic locationand radius information in the request, to identify all mobile stationsin the desired vicinity (block 122). The identity and geographicinformation associated with each mobile station satisfying the searchcriteria is then sent as a “snapshot response” message (block 124) backto mobile switching center 20.

As an alternative to the process used with the architecture of FIG. 10,each mobile station 12 may periodically determine its geographiclocation and store this information (and the time of the recording) inan internal circular buffer within mobile station 12 itself. Thecircular buffer is preferably formed to include a plurality of Nregisters, and is therefore capable of storing a history of the last Nlocations of mobile station 12. Thus, as new location information iswritten in the circular buffer, the “oldest” information is discarded.FIG. 17 depicts an exemplary message sequence associated with thisarrangement. As before, the process starts with a particular mobilestation 12 making a 911 to a PSAP 30 (message sequence (1)). The agentat PSAP 30 determines that it would be valuable to identify witnessesassociated with this 911 call, and sends a “snapshot request” (messagesequence (2) to position server 52. The format of this “snapshotrequest” is the same as that discussed above with the other embodiments.Position server 52 then sends an “acknowledgement” (message sequence(3)), including a unique transaction ID, back to PSAP 30, and at thesame time forwards the “snapshot request” to one or more mobileswitching centers 20 (message sequence (4)), in the manner discussedabove.

In contrast to the previously discussed embodiment, in this case eachmobile switching center 20 then broadcasts the snapshot request to allmobile stations 12 in its coverage area (message sequence (5)). Eachmobile station 12 then searches its own circular buffer and determines,using the time, geographic location information, and radius of search,if it was within the requested witness area. If so, it waits a randomamount of time and then forwards its identifying information as a“snapshot response” to mobile switching center 20 (message sequence(6)). By including a random delay in the response, there is an increasedassurance that mobile switching center 20 will receive all “snapshotresponse” messages without overloading the system and blocking someresponse messages. Once all of the responses have arrived, mobileswitching center forwards this information as its “snapshot response”message (message sequence (7)) to position server 52.

Flowcharts illustrating the particular processes occurring within mobilestation 12 for this embodiment are shown in FIGS. 18 and 19. Inparticular, FIG. 18 illustrates an exemplary set of process stepsassociated with the process of updating the geographic location of anexemplary mobile station 12. The process begins (block 150) with mobilestation 12 determining its current geographic position. Any appropriatearrangement for providing such a location feature may be used, such as,for example, the Global Positioning System (GPS). Mobile 12 thenaccesses an internal clock to determine the current date and time (block152). Before writing this updated location, date and time informationinto its circular “location” buffer, a check is determined to see if thecircular buffer is “locked” (step 154), meaning that the data has beenfrozen and permission to overwrite the data has been denied. As will bediscussed below, the buffer information is “locked” upon receipt of a“snapshot request” by mobile station 12. If indeed the buffer is locked,the process will jump to block 156, wait S seconds and go back to step150 and re-determine its current position. If the buffer is not locked,the collected location, time and data information is sent to themobile's “location” buffer and overwrites the oldest stored locationinformation (block 158).

The process used by a mobile station 12 in responding to a broadcasted“snapshot request” is contained in the flowchart of FIG. 19. As shown,the process begins by the receipt of the “snapshot request” at mobilestation 12 (block 160). When a request is received, mobile station 12then “locks” its location buffer (block 162) to prevent further updatesfrom occurring (insuring that the current location information stored inits buffer would be most relevant to the “snapshot request”). Afterlocking the circular buffer, a query is made to determine if any of theinformation stored in the buffer satisfies the “snapshot request” interms of the queried “time” and “radius” of the search around a certaingeographic location (block 164). If the stored information does notsatisfy the criteria of the “snapshot request”, the location buffer isunlocked (block 166), and the process ends. If the stored informationdoes satisfy the request, then mobile station 12 “waits” a random timeinterval between S₁ and S₂ seconds (block 168), and then transmits itsidentity and relevant location information back to mobile switchingcenter 20 (block 170). The inclusion of a random delay before respondingminimizes the chance of overloading both base station subsystem 14 andmobile switching center 20 with the responses from multiple mobilestations. Once this information has been transmitted, the mobile'sbuffer is unlocked (block 166), and the process ends.

While there have been illustrated and described what are considered tobe preferred embodiments of the present invention, it will be understoodby those skilled in the art that various changes and modifications canbe made, and equivalents may be substituted for elements thereof withoutdeparting from the true scope of the present invention. In addition,many modifications may be made to adapt a particular situation to theteaching of the present invention without departing from the centralscope thereof. Therefore, it is intended that the present invention notbe limited to the particular embodiments disclosed as the best modecontemplated for carrying out the present invention, but that thepresent invention includes all embodiments falling within the scope ofthe claims appended hereto.

What is claimed is:
 1. In a mobile communication system, an arrangementfor identifying and collecting relevant witness information associatedwith a 911 call from a mobile station, the system comprising a pluralityof base station subsystems for receiving voice communications frommobile stations; a plurality of mobile switching centers, eachresponsive to communications from a subset of said plurality of basestation subsystems; at least one public safety answering position,coupled to the plurality of mobile switching centers through acommunication network; a position server coupled to the at least onepublic safety answering position through a data network, said positionserver for collecting geographic location information of other mobilestations in a predetermined area surrounding the geographic location ofa mobile station placing a 911 call and storing the informationregarding each identified mobile station near the geographic location ofthat call in a transaction table, wherein each base station subsystemincludes a receive-only antenna for collecting geographic positioninformation from each transmitting mobile station and forwarding thegeographic position information directly to the position server forstorage in a geographic location database.
 2. A communication system asdefined in claim 1 wherein each mobile station forwards its geographiclocation information to its associated mobile switching center, theplurality of mobile switching centers being connected through the datanetwork to the position server and the position server including a tableidentifying each mobile switching center and its coverage area, suchthat upon receiving a request for information the position serverqueries relevant mobile switching centers which then forward theidentity and location of all mobile stations which meet the searchcriteria.
 3. A communication system as defined in claim 2 wherein eachmobile station sends its geographic location information to itsassociated mobile switching center when transmitting.
 4. A communicationsystem as defined in claim 2 wherein each mobile station sends itsgeographic location information to its associated mobile switchingcenter at predetermined time intervals, whether or not engaged incommunication.
 5. A communication system as defined in claim 1 whereineach mobile station includes a buffer for storing its own geographiclocation information such that when the position server receives arequest for information the request is forwarded to relevant mobileswitching centers to broadcast to all its associated mobile stations andthen collect the relevant information.
 6. A communication system asdefined in claim 1 wherein the stored geographic location information isderived using a time difference of arrival calculation method.
 7. Acommunication system as defined in claim 1 wherein the stored geographiclocation information is derived using an angle of arrival calculation.8. A communication system as defined in claim 1 wherein the storedgeographic location information is derived using an RF fingerprintingmethod.
 9. A communication system as defined in claim 1 wherein thestored geographic location information is derived using a globalpositioning system.
 10. A method of determining the identity of mobilestations in a communication network that are in a predetermined vicinityof a 911 call from a mobile station, the method comprising the steps of:a) determining, at a public safety answering position, that a 911 callin progress requires witness identification; b) preparing a “snapshotrequest” at the public safety answering position, the “snapshot request”including the phone number of the 911 mobile station, its geographiclocation and a radius of search, using the geographic location as acenter; c) sending the “snapshot request” from the public safetyanswering position to a position server in the communication network; d)determining the identity of all mobile stations satisfying the criteriaof the “snapshot request” by; collecting, at a mobile switching center,geographic location information for all transmitting mobile stations incommunication with said mobile switching center; creating a database atsaid mobile switching center for storing the identity of eachtransmitting mobile station and its current geographic locationinformation; in response to receiving a request from the positionserver, searching the database at said mobile switching center to findthe identity of transmitting mobile stations satisfying the searchcriteria; and forwarding to the position server the identity andlocation information of all mobile stations satisfying the searchcriteria; and e) creating a transaction table of all mobile stationsdetermined in step d).
 11. A method of determining the identity ofmobile stations in a communication network that are in a predeterminedvicinity of a 911 call from a mobile station, the method comprising thesteps of: a) determining, at a public safety answering position, that a911 call in progress requires witness identification; b) preparing a“snapshot request” at the public safety answering position, the“snapshot request” including the phone number of the 911 mobile station,its geographic location and a radius of search, using the geographiclocation as a center; c) sending the “snapshot request” from the publicsafety answering position to a position server in the communicationnetwork; d) determining the identity of all mobile stations satisfyingthe criteria of the “snapshot request” by; collecting, at predeterminedtime intervals at a mobile switching center, geographic locationinformation for all mobile stations in the serving area of said mobileswitching center, whether or not the mobile stations are transmittingvoice communications; creating a database at said mobile switchingcenter for storing the identity of each mobile station and its currentgeographic location information; in response to receiving a request fromthe position server, searching the database at said mobile switchingcenter to find the identity of all mobile stations satisfying the searchcriteria; and forwarding to the position server the identity andlocation information of all mobile stations satisfying the searchcriteria; and e) creating a transaction table of all mobile stationsdetermined in step d).
 12. A method of determining the identity ofmobile stations in a communication network that are in a predeterminedvicinity of a 911 call from a mobile station, the method comprising thesteps of: a) determining, at a public safety answering position, that a911 call in progress requires witness identification; b) preparing a“snapshot request” at the public safety answering position, the“snapshot request” including the phone number of the 911 mobile station,its geographic location and a radius of search, using the geographiclocation as a center; c) sending the “snapshot request” from the publicsafety answering position to a position server in the communicationnetwork; d) determining the identity of all mobile stations satisfyingthe criteria of the “snapshot request” by; creating a location buffer ineach mobile station; updating, at predetermined intervals, thegeographic location information of each mobile station and storing thegeographic location information in its associated buffer; in response toreceiving a “snapshot request” at the position server, forwarding the“snapshot request” to each relevant mobile switching center;broadcasting, at each relevant mobile switching center, the “snapshotrequest” to all mobile stations in its serving area; searching thelocation buffers in each mobile station and transmitting mobile stationidentity and geographic location information from mobile stationssatisfying the search criteria to their associated mobile switchingcenters; and forwarding the responding information from said mobileswitching centers to the position server; and e) creating a transactiontable of all mobile stations determined in step d).